The present invention relates to a data processing device provided with a microprocessor that functions in response to signals produced by a clock oscillator to change the data-processing speed of the microprocessor. When a high-speed processor incorporating a microprocessor is used with electronic game software programs that have been designed for use with a low-speed processor, the execution keys are normally unable to operate quickly enough to match the game's movements. Key-input operation cannot be properly executed because the stand-by time for key-input operation is too short. To compensate for this, conventionally, a variety of systems are made available for changing the clock frequency of the microprocessor. However, this makes the hardware rather expensive.